Alkaline-earth aluminoborosilicate glass and the uses thereof

ABSTRACT

The invention relates to low-alkali or alkali-free alkaline-earth aluminoborosilicate glasses of the following composition (in wt.-% on an oxide basis) SiO 2 &gt;49-65; B 2 O 3  0.5-4.5; Al 2 O 3 &gt;10-23; MgO&gt;2.7-7; CaO 0.5-10; SrO&gt;15-22; BaO 0.5-7; provided that MgO+CaO+SrO+BaO&gt;20-35; SnO 2  0-2; ZrO 2  0-2; ZrO 2  0-2; TiO 2 0-2; CeO 2 0-1.5; ZnO 0-1; Na 2 O 0-2; K 2 O 0-2; provided that Na 2 O+K 2 O equals 0-3. The inventive glasses are especially suitable for use as substrates in thin film photovoltaic technology and as glasses for bulbs.

The invention relates to aluminoborosilicate glasses which containalkaline earth metals. The invention also relates to uses of theseglasses.

When energy is being obtained by means of photovoltaics, the property ofcertain semiconducting materials of absorbing light from the visiblespectral region as well as the near UV or IR to form free chargecarriers (electron/hole pairs) is utilized. If there is an internalelectric field in the solar cell, produced by a pn junction in thephotoactive semiconductor material, these pairs can be spatiallyseparated using the diode principle, leading to a potential differenceand, given suitable contacts, to the flow of current. Solar cell systemswhich are currently commercially available contain, as photoactivematerial, almost exclusively crystalline silicon. This is produced aswhat is known as “solar grade Si”, inter alia, as a waste materialduring the production of high-purity silicon single crystals for complexintegrated components (chips).

The possible applications for photovoltaic installations can be roughlydivided into two groups. These are, firstly, applications which are notconnected to the mains, which are used in remote areas on account of thelack of energy sources which are relatively easy to install. Bycontrast, solutions which are connected to the mains and in which solarenergy is fed into an existing fixed mains remain uneconomical, onaccount of the high cost of solar current, and are therefore relativelyrare.

Therefore, the future market development of photovoltaics, in particularfor solutions which are connected to the mains, is highly dependent onthe potential for reducing costs in the production of solar cells. Theimplementation of thin-film concepts is considered to offer greatpotential. In this case, photoactive semiconductor materials, inparticular highly absorbent compound semiconductors, are deposited inlayers which are a few μm thick on substrates which are as inexpensiveas possible and are able to withstand high temperatures, for exampleglass. The possibilities for reducing costs lie primarily in the lowconsumption of semiconductor material and the excellent possibilitiesfor automation during the production compared to wafer Si solar cellproduction.

Solar cells based on the II-VI compound semiconductor CdTe are apromising thin-film concept. This material satisfies essentialconditions, such as a band gap which is well matched to the solarspectrum, high absorption of the incident light and very good chemicalstability of the compound.

The same is true of the compound semiconductor Cu(In,Ga) (S,Se)₂,(“CIS”). Compared to the first example, this is also moreenvironmentally friendly, since it does not contain any Cd.

Thin polycrystalline films of CdTe can be produced by a range of methods(vapor deposition, screen printing, sublimation, spray pyrolysis,electrodeposition), but only in p-conducting form. To obtain a pnjunction, what is known as a heterojunction has to be produced using adifferent n-conducting material, e.g. CdS.

In addition to the substrate technologies which are in widespread use inthin-film photovoltaics (semiconductor resting on bases made frommaterials such as glass, metal, plastic, ceramic), having said layersand a covering glass, with the light acting through the covering glass,a superstrate arrangement has also established itself in particular inCdTe photovoltaics. In this arrangement, the light from impingement onthe semiconductor layer initially passes through the support material.This eliminates the need for the covering glass, which has advantages interms of costs. To achieve high efficiencies, it is necessary forsubstrates of this type to have a high transparency in the VIS/UV regionof the electromagnetic spectrum, which makes the use of glass a suitablesolution. For example, even semitransparent glass ceramics areunsuitable, partially for cost reasons caused by the ceramicizingprocess.

Further demands on the substrate/superstrate material result from thestructure of the solar cell and the temperature conditions during theprocess used for deposition of the CdTe film. With a view to achievingrapid deposition rates for good-quality CdTe, high temperatures,generally of over 650° C., are required. Accordingly, the substrateglasses should have a sufficiently high ability to withstand thermalloads, i.e. the transformation point T_(g) of the glasses should be over660° C. To prevent flaking of the semiconductor layer during the coolingwhich follows the coating process, the glasses must also be matched tothe thermal expansion of CdTe (α_(20/300)≈5-6*10⁻⁶/K). In the case ofthe CIS technology, in addition to the high T_(g) (>650° C.), acoefficient of thermal expansion α_(20/300) which is matched to the Molayer functioning as electrode, of 4.5-5.0*10⁻⁶/K, is required. Thesoda-lime glass which has previously been used in no way satisfies theserequirements, having an α_(20/300)≈9*10⁻⁶/K and a T_(g) of approx. 520°C.

Furthermore, the glasses are to be sufficiently mechanically stable andchemically resistant to water and also to any reagents used in theproduction process, in particular in the; case of the superstrateconcept, in which there is no covering glass protecting the solar modulefrom environmental influences. For example, soda-lime glasses only havea hydrolytic resistance belonging to Hydrolytic Class 3. Furthermore, itshould be possible to economically produce the glasses in sufficientquality in terms of having no or few bubbles and crystalline inclusions.

Similar demands are also imposed on glasses for lamp bulbs:

The glasses have to be able to withstand high thermal loads, since highbulb temperatures generally occur in operation. The glasses must besufficiently resistant to devitrification to be suitable for tubedrawing. For use as lamp bulb glass for lamp bulbs which includemolybdenum components as electrode or supply conductor material, thethermal expansion of the glasses has to be matched to that of molybdenum(α_(20/300)=5.0*10⁻⁶/K), so that a sealed, stress-free fusion betweenthe metal and the glass is achieved. For this application too, theglasses must be as free from bubbles as possible. Moreover, glasses forhalogen lamps must be substantially free of alkali metals, since alkalimetal ions disrupt the regenerative halogen cycle of the lamp.

This profile of requirements is best satisfied by aluminoborosilicateglasses which contain alkaline earth metals but little if any alkalimetal. However, the known glasses for display or solar cell substrateswhich are described in the following documents still have drawbacks interms of their chemical and physical properties and/or their formationoptions and fail to satisfy the full range of demands.

Numerous documents describe glasses with relatively high B₂O₃ contents,for example DE 196 01 922 A, JP 58-120 535 A, JP 60-141 642 A, JP 8-295530 A, JP 9-169 538 A, JP 10-59 741 A, JP 10-722 37 A, EP 714 862 A1, EP341 313 B1, U.S. Pat. No. 5,374,595, DE 197 39 912 C1. These glasses donot have the required high transformation temperatures and/or havecoefficients of expansion which are too low for the applications whichare preferred in this document.

By contrast, B₂O₃-free glasses are described in U.S. Pat. No. 4,607,016,JP 61-236 631 A and JP 61-261 232 A. The absence of B₂O₃ means that theglasses are difficult to melt and tend towards devitrification. Theglasses mentioned in WO 97/30001 also do not contain any B₂O₃.

DE 44 30 710 C1 describes borosilicate glasses with a low boric acidcontent and high SiO₂ contents (>75% by weight) which means that theyare highly viscous even at high temperatures and can only be melted andrefined at considerable cost. Moreover, these glasses, withtransformation temperatures T_(g) of between 500 and 600° C., have onlya relatively low thermal stability.

DE 196 17 344 C1 and DE 196 03 698 C1, in the name of the applicant,have disclosed alkali-free, tin-containing glasses with a coefficient ofthermal expansion α20/300 of approx. 3.7*10⁻⁶/K and very good chemicalstabilities. They are suitable for use in display technology. However,since they inevitably contain at least 1 to 2% by weight of the networkmodifier ZnO, they are not optimally suitable in particular forprocessing on a float installation.

The Pb-containing glasses which also have a relatively high Zn content(≧3.5% by weight) described in JP 61-295 256 A are also ratherunsuitable for the float process, since, if the concentration is toohigh, it is easy for deposits of ZnO and PbO or Pb to form on the glasssurface in the reducing forming-gas atmosphere as a result ofevaporation followed by condensation.

Transparent glass ceramics, which are suitable, inter alia, for flatdisplays and solar cells, are described in JP 3-164445 A. The exampleslisted have high T_(g) values of >780° C. and are well matched to CdTein terms of their thermal expansion. However, on account of their veryhigh zinc contents they are unsuitable for the float production process.The same applies to the transparent mullite-containing glass ceramics,which are doped with at most 1% by weight of chromium, from EP 168 189A2 and the transparent garnet glass ceramics from JP 1-208343 A withpossible applications in solar collectors. However, the hightransparency required for use as superstrate in CdTe solar cell systemsis not ensured either by glass ceramics, which, depending on the grainsize of the crystallites, have a transmission which is lower than thatof glasses, or by milky-white opal glasses as are described in FR2126960.

JP 9-48632 A describes alkali-free aluminoborosilicate glasses whichcontain alkaline earth metals, with maximum alkaline earth metal oxidecontents of 20% by weight. These glasses also have coefficients ofthermal expansion which are too low for the applications preferred inthis document. DE 196 80 966 T1 and DE 196 80 967 T1 describealkali-free glass substrates which contain little or no MgO. Theirglasses with preferred alkaline earth metal contents of at most 20% byweight also have an insufficiently high expansion.

By contrast, glasses which contain little or no alkali metal but have anexcessively high expansion are known from WO 96/9259, JP 9-255354 A, JP9-255355 A, JP 9-255 356 A and U.S. Pat. No. 5,741,746. The glassesdescribed in WO 96/9259, with at least 45.5% by weight of RO (at least25.5% by weight of BaO) have a high alkaline earth metal oxide content,as do those described in JP 9-255 354 A (BaO≧8% by weight) and U.S. Pat.No. 5,741,746 (BaO≧14% by weight), while the glasses described in JP9-255 355 are low in SiO₂.

A substrate glass for applications in LCD and solar cell technology isdescribed in U.S. Pat. No. 4,994,415. The glass, which does not containany alkali metals or magnesium, includes high levels of BaO, at morethan 10% by weight, and therefore ought to be highly resistant todevitrification. However, the density of the glass is high on account ofthis component. Similar statements are true of the BaO-rich glassesdescribed in U.S. Pat. No. 5,326,730 (12-19 mol % of BaO).

If the BaO contents are too low, there is an increased tendency towardcrystallization. This is true not only of the glasses described in WO98/27019, which contain <3% by weight of SrO+BaO, but also, inparticular, of the glasses described in EP 510 544 B1, which contain nobarium and have a difference between the working temperature(temperature at the viscosity 10⁴ dPas) and the upper devitrificationlimit (liquidus temperature) which is unfavorable for the productionprocess. JP 10-45422 A and JP 9-263 421 A describe glasses with alkalineearth metal contents of at most 20 or 22 mol %, respectively, and BaOcontents of at most 1 mol %, preferably without any BaO, these glassesalso having relatively low SrO contents. The glasses have very hightemperatures at the viscosities 10⁴ dpas and 102 dPas, which places veryhigh demands on the tank furnace and manifold material, so that theglasses cannot be produced at low cost. The glasses described in JP4-175242 A also have relatively low SrO contents (1-9 mol %), but theirBaO contents, which are likewise up to 9 mol %, are also rather high.

JP 10-25132 A describes glasses which are refined by means of a combinedsulfate/chloride refining. The compositions vary over a wide range butinclude only at most 10% by weight of the component SrO, which is onlyoptional as is the case for all alkaline earth metal oxides. Their B₂O₃content is very high, at up to 20% by weight.

DE-A 1596 767 has already described aluminosilicate glasses fortungsten-iodine lamps which are substantially free of alkali metaloxides. The glasses have an alkaline earth metal oxide content ofbetween 10 and 25% by weight, which can be combined as desired from MgO,CaO, SrO, BaO. According to the examples, it is composed of CaO and BaOand, if appropriate, MgO.

The glasses described in JP 1-126239 A may also vary considerably interms of the content of the constituent CaO and the optionalconstituents SrO and BaO, the sum of these three components ranging from12-25 mol %. The molar ratio between the sum of the said alkaline earthmetal oxides, on the one hand, and this sum+Al₂O₃, on the other hand, isgreater than 0.4 and less than 0.6.

On account of their high ratio of network-forming agents to alkalineearth metal oxides in combination with low B₂O₃ contents, thealkali-free glasses described in EP 0 528 149 B1 should be sufficientlythermally stable for high-temperature coating processes. With alkalineearth metal oxide contents of between 23 and 28 mol %, they encompass arange which can be used to achieve widely varying expansioncoefficients. The dominant alkaline earth metal oxide is CaO, while MgOis not present or is present in only small amounts.

U.S. Pat. No. 5,116,789 and EP 0 527 320 B1 have disclosed MgO-freeglasses, the dominant alkaline earth metal oxide in which is SrO (15-26or 21-26 mol %, respectively) JP 9-12333 A has disclosed glasses forhard disk substrate which are rather low in SrO and high in CaO. Thealkali-free aluminosilicate glasses for flat displays which aredescribed in EP 672 629 A2 and U.S. Pat. No. 5,508,237 have similarmaximum SrO contents. These documents show various composition rangeswith different coefficients of thermal expansion. Allegedly, it issupposed to be possible to produce the glasses not only by theoverflow-fusion drawing process but also by the float process, yet thisis not possible using the refining agents As₂O₃ and Sb₂O₃ which arementioned by way of example and the optional glass components Ta₂O₅ andNb₂O₅, on account of the ease with which they can be reduced.

The sodium-containing glasses described in JP 4-83733 A comprise atleast 80% by weight of SiO₂, Al₂O₃, Na₂O and MgO. With this basiccomposition, it is not possible to achieve a thermal expansion ofapprox. 5-6*10⁻⁶/K with, at the same time, a high transformationtemperature, or else it is possible to achieve this only by acceptingdrawbacks in terms of other properties.

It is an object of the invention to provide glasses which satisfy theabovementioned physical and chemical demands imposed on glass substratesfor thin-film photovoltaic technologies based on compoundsemiconductors, in, particular based on the II-VI semiconductor CdTe orCIS, glasses which have a thermal stability which is sufficient forhigh-temperature deposition processes, i.e. a transformation temperatureT_(g) of at least 660° C., which have a working temperature range whichis appropriate to the process and have a high devitrification stabilityand also a high quality with regard to the low level of bubbles and achemical stability which at least matches that of soda-lime glasses.

This object is achieved by the aluminoborosilicate glasses which containalkaline earth metals in accordance with claim 1.

The glasses contain balanced levels of the network-forming agents SiO₂and Al₂O₃, with relatively small amounts of the network-forming agentB₂O₃. In this way, a high thermal stability of the glass combined withmelting and working temperatures which remain low is achieved. Toachieve the desired coefficient of thermal expansion (α_(20/300) between4.5 and 6.0*10⁻⁶/K), the maximum content of SiO2+Al₂O₃+B₂O₃ is at most<80% by weight, preferably at most 74% by weight.

IN DETAIL

The glasses contain >49-65% by weight of SiO₂, preferably 50-64% byweight. Lower levels lead to a deterioration in the chemical stability,in particular the acid resistance, of the glass, while higher levelscause the thermal expansion to become excessively low. Moreover, in thelatter case an increase in devitrification tendency is observed.

The glasses contain >10-23% by weight, preferably >10-22% by weight ofAl₂O₃. A higher level has adverse effects on the process temperaturesfor hot-forming, while excessively low levels may make the glass moresusceptible to crystallization.

The glasses contain 0.5-4.5% by weight of B₂O₃. The desired hightransformation temperature is ensured by limiting the maximum B₂O₃content. Furthermore, the low level of boric acid has positive effectson the chemical stability of the glass, in particular with respect toacids. However, it is not advisable to eliminate boric acid altogether,since it facilitates melting; even low levels of >0.5% by weight maketheir presence felt in a positive manner both in the melt flow and inthe crystallization behavior.

The desired coefficient of thermal expansion α_(20/300) of between4.5*10⁻⁶ and 6.0*10⁻⁶/K can be achieved with an alkaline earth metaloxide content of >20 to 35% by weight, preferably up to 32% by weight,particularly preferably of more than 26% by weight, by means of amultiplicity of combinations of the individual alkaline earth metaloxides.

Glasses with low expansion coefficients (α_(20/300)≦5.0*10⁻⁶/K) tend tocontain smaller amounts of alkaline earth metal oxides,preferably >20-28% by weight, while glasses with higher expansioncoefficients α_(20/300) have relatively high alkaline earth metal oxidecontents.

The respective levels of the individual alkaline earth metal oxides, andtherefore their ratio to one another, are significant, since high levelsof the light oxides MgO and. CaO would be advantageous with a view toachieving the lowest possible working temperature V_(A) and a lowdensity, but excessively high levels of these two oxides with relativelysmall cations, in particular of MgO, increases the likelihood ofsegregation, which has adverse effects on the spectral transmission andchemical stability of the glasses, and since high BaO contents increasethe resistance to crystallization and thermal stability of the glasses,but result in glasses of undesirably high density, while SrO contentsimprove the resistance to crystallization and increase thetransformation temperature without an excessively high rise in thedensity.

Consequently, it has proven optimal for SrO to be dominant, specificallyfor the glass to contain >2.7-7% by weight of MgO, 0.5-10% by weight ofCaO, particularly preferably 0.5-9% by weight of CaO, >15.5-22% byweight of SrO, 0.5-7% by weight of BaO, particularly preferably at least2.5% by weight of BaO, very particularly preferably at most <5% byweight of BaO.

The glasses may contain small amounts of alkali metal oxides,specifically up to 2% by weight of Na₂O and up to 2% by weight of K₂O,with Na₂O+K₂O=0-3% by weight. These oxides improve the ease of melting.Furthermore, it has been found that, when using the CIS technology, thepresence of small quantities of alkali metal oxides improves theefficiency of the solar cell. Higher levels of alkali metal oxides wouldcause the thermal expansion to increase excessively and would reduce thetransformation temperature.

It is preferable for alkali metal oxides not to be added.

However, small amounts of Na₂O and K₂O, i.e. in particular amounts ofapprox. one tenth of a percent by weight, as result from an alkali metalhalide refining of the melt, can be present in the glasses forphotovoltaic applications without there being any problems.

Particularly for use as lamp glasses, alkali metal halide refining isnot used, and the glasses are free of alkali metal oxides apart frominevitable impurities.

On account of the high alkaline earth metal content (>20% by weight,preferably >26% by weight) and a maximum level of 7% by weight of MgO,the sum of the glass-forming agents SiO₂, Al₂O₃, the alkali metal oxideNa₂O and MgO is relatively low. It may be at most 86.5% by weight, butis preferably kept to <80% by weight. This ensures that there aresufficient amounts of heavy alkaline earth metal oxides and that thedesired properties with regard to thermal expansion and transformationtemperature are simultaneously achieved.

On account of the high alkaline earth metal content, the ratio of thealkaline earth metal oxides to the sum of alkaline earth metal oxidesand Al₂O₃ is relatively high. Its molar ratio is preferably more than0.6.

The glass may contain up to 1% by weight of ZnO. Since its influence onthe viscosity characteristic curve resembles that of boric acid, ZnO onthe one hand has the effect of loosening the network, but on the otherhand does not increase the thermal expansion to the same degree as thealkaline earth metal oxides. Particularly when the glasses are processedin the float process, the ZnO content is preferably limited torelatively small amounts (≦0.5% by weight), or ZnO is dispensed withaltogether. Amounts of greater than 0.5% by weight increase the risk ofdisruptive ZnO deposits on the glass surface. These may form throughevaporation and subsequent condensation in the hot-forming area.

The glass may contain up to 2% by weight, preferably up to 1.5% byweight of ZrO₂. ZrO₂ increases the thermal stability of the glass.However, levels or more than 2% by weight may lead to the formation ofmelting residues in the glass, on account of the difficulty ofdissolving ZrO₂. It is preferable for at least 0.1% by weight of ZrO₂ tobe present.

The glass may contain up to 2% by weight, preferably up to 1.5% byweight of TiO₂. TiO2 reduces the tendency of the glasses towardsolarization. Levels or more than 2% by weight may cause discolorationson account of the formation of complexes with Fe³⁺ ions. It ispreferable for at least 0.1% by weight of TiO₂ to be present. The glassmay contain up to 2% by weight, preferably up to 1.5% by weight, ofSnO₂. SnO₂ is a highly effective refining agent in particular inhigh-melting, alkali-free aluminoborosilicate glass systems whichcontain alkaline earth metals. The tin oxide is used as SnO₂, and itstetravalent state is stabilized by the addition of other oxides, such asfor example TiO₂, or by the addition of nitrates. Since it is difficultto dissolve at temperatures below the working temperature V_(A), theamount of SnO₂ is restricted to the upper limit given above. This avoidsprecipitation of microcrystalline Sn-containing phases. It is preferablefor at least 0.1% by weight of SnO₂ to be present.

The glass may also contain up to 1.5% by weight, preferably up to 1.0%by weight of CeO₂. It is preferable for there to be at least 0.1% byweight of CeO₂. The combination of SnO₂ with CeO₂ stabilizes theSnO₂/SnO redox equilibrium, and the very good refining action of theSnO₂ is improved still further. However, CeO₂ is also highly effectiveas the sole refining agent in high-melting glasses. The CeO₂ content ispreferably limited to at most 0.5% by weight.

Glasses which melt at relatively low temperatures can also be refinedusing alkali metal halides. For example, sodium chloride contributes torefining on account of its evaporation at over approx. 1410° C., some ofthe NaCl used making its presence felt again as Na₂O in the glass. If1.5% by weight of NaCl is added, approx. 0.1% by weight of Cl⁻ remainsin the glass.

The alkali metal ions which are present in the glass do not have anadverse effect on the semiconductor layer comprising the compoundsemiconductors CdTe and CIS.

The glasses can be processed to form flat glasses using the variousdrawing processes, e.g. microsheet-down-draw, up-draw or overflow-fusionprocesses.

The glass may contain up to 1.5% by weight of As₂O₃ and/or Sb₂O₃ asadditional or sole refining agent(s). It is also possible to add in eachcase 1.5% by weight of Cl⁻ (for example as BaCl₂ or NaCl), F⁻ (e.g. asCaF₂ or NaF) or SO₄ ²⁻ (e.g. as BaSO₄). However, the sum of As₂O₃,Sb₂O₃, Cl⁻, F⁻ and SO₄ ²⁻ should not exceed 1.5% by weight. If therefining agents As₂O₃ and Sb₂O₃ are not present, the glass can also beprocessed using the float process.

EXEMPLARY EMBODIMENTS

Glasses were melted from conventional raw materials at 1560° C. in Pt/Ircrucibles. The melt was refined for one hour at this temperature and wasthen transferred into inductively heated platinum crucibles and agitatedfor 30 minutes at 1540° C. for homogenization.

The table shows six examples of glasses according to the invention,giving their compositions (in % by weight based on oxide) and their mostimportant properties, including:

the density ρ {g/cm³}

the coefficient of thermal expansion α_(20/300) [10⁻⁶/K]

the dilatometric transformation temperature T_(g)[° C.] in accordancewith DIN 52324

the temperature at the viscosity 10² dPas (given as T 2 [° C.]),calculated from the Vogel-Fulcher-Tamman equation

the temperature at the viscosity 10⁴ dPas (denoted as T 4 [° C.])

the temperature at viscosity 10^(7.6) dPas (denoted as T 7.6 [° C.])

the temperature at viscosity 10¹³ dPas (denoted as T 13 [° C.])

the temperature at viscosity 10^(14.5) dPas (denoted as T 14.5 [° C.]),calculated from the Vogel-Fulcher-Tammann equation

the hydrolytic stability in accordance with DIN ISO 719 “H” [μg Na₂O/g].With a base equivalent as Na₂O per g of glass grit of ≦31 μg/g, theglasses belong to Hydrolytic Class 1 (“chemically highly resistantglass”)

the acid resistance in accordance with DIN 12166 “S” [mg/dm²]. With aweight loss of over 0.7 to 1.5 mg/dm², the glasses belong to Acid Class2, and with a weight loss of over 1.5 to 15 mg/dm² the glasses belong toAcid Class 3

the lye resistance in accordance with ISO 695 “L” [mg/dm²]. With aweight loss of over 75 to 175 mg/dm², the glasses belong to Lye Class 2

the maximum transmission at wavelengths of between 400 and 850 nm(specimen thickness 2.5 mm) τ_(max) (400-850 nm)

the upper devitrification limit OEG [° C.], i.e., the liquidustemperature

the maximum crystal growth rate V_(max) [μm/h]

TABLE Compositions (in % by weight based on oxide) and significantproperties of glasses according to the invention. 1 2 3 4 5 6 SiO₂ 58.852.3 52.5 50.8 58.8 51.0 B₂O₃ 1.0 2.9 1.1 4.0 1.0 0.5 Al₂O₃ 10.5 16.514.0 12.8 10.5 20.0 MgO 5.0 2.8 4.0 6.0 4.2 3.5 CaO 5.0 5.3 4.0 1.0 5.03.7 SrO 16.0 17.0 16.0 20.0 16.0 16.0 BaO 3.0 2.5 7.0 5.0 3.0 4.0 ZrO₂0.5 0.3 1.4 0.2 0.5 0.3 SnO₂ 0.2 0.4 — 0.2 — — Na₂O — — — — 1.0 — K₂O —— — — — 1.0 ρ [g/cm³] 2.810 2.813 2.896 2.883 2.790 2.830 α_(20/300)5.34 5.20 5.39 5.45 5.75 5.40 [10⁻⁶/K⁻¹] Tg [° C.] 728 735 740 706 689748 T 14.5 [° C.] 703 708 n.d. n.d. n.d. n.d. T 13 [° C.] 734 739 n.d.n.d. 734 n.d. T 7.6 [° C.] 919 925 n.d. n.d. 896 n.d. T 4 [° C.] 12061210 1207 1157 1178 1251 T 2 [° C.] 1549 1549 n.d. 1495 1585 1598 H [μgNa₂O/g] 18 13 n.d. n.d. 18 n.d. S [mg/dm²] 1.5 n.d. n.d. n.d. n.d. n.d.L [mg/dm²] 91 n.d. n.d. n.d. n.d. n.d. τ_(max) (400-850 nm) 90.0 90.2n.d. n.d. 90.5. n.d. OEG [° C.] 1200 n.d. n.d. n.d. 1140 n.d. V_(max) 17n.d. n.d. n.d. 22 n.d. n.d. = not determined

As the exemplary embodiments illustrate, the glasses according to theinvention have the following advantageous properties:

A thermal expansion α_(20/300) of between 4.5*10⁻⁶/K and 6.0*10⁻⁶/K,therefore matched to the expansion behavior of CdTe or to the Mo layerwhich is applied as electrode in the CIS technology.

With T_(g)>660° C., a very high transformation temperature. This is ofessential importance for the shrinkage (compaction) caused by productionto be as low as possible and for the glasses to be used as substrates inhigh-temperature deposition processes. The high transformationtemperature explains the high ability of the glasses to withstandthermal loads.

A temperature at viscosity 10⁴ dPas of at most 1300° C., which indicatesa working range which is appropriate to the process, and good resistanceto devitrification. These two properties enable the glass to be producedas flat glass using the various drawing processes, e.g.microsheet-down-draw, up-draw or overflow-fusion processes, and, in apreferred embodiment, if it is free of As₂O₃ and Sb₂O₃, also using thefloat process.

A very high hydrolytic stability.

Furthermore, the glasses have a high resistance to solarization, a hightransparency (≧90% τ_(max) (400-850 nm; 2.5 mm)) and a high quality interms of having few if any bubbles.

Consequently, the glasses are eminently suitable for use as substrateglass in thin-film photovoltaics, especially based on compoundsemiconductors, in particular based on CdTe and on Cu(In,Ga) (Se,S)₂(CIS).

Glasses with coefficients of thermal expansion α_(20/300) of between 4.5and 5.0*10⁻⁶/K are used for the CIS technology. In terms of theirexpansion behavior, they are matched to that of the Mo layers used aselectrode. Glasses whose thermal expansion is matched to that of CdTe,i.e. which have an α_(20/300) of between 5.0 and 6.0*10⁻⁶/K, are usedfor solar cells based on CdTe. The specific a values can be achieved byvarying the RO content.

Glasses with transformation temperatures T_(g) of >715° C. and withcoefficients of thermal expansion α_(20/300) of between 4.5 and5.0*10⁻⁶/K are eminently suitable for use as glasses which are to befused to Mo, since their thermal expansion is matched to that ofmolybdenum and they have a very high ability to withstand thermal loads,and if they are free of alkali metals, they are also eminently suitableas bulb glass for lamp bulbs which have Mo components, in particular forthose which have bulb temperatures of approx. 550-640° C.

What is claimed is:
 1. An aluminoborosilicate glass which containsalkaline earth metals which has the following composition (in % byweight, based on oxide): SiO₂ >49-65 B₂O₃ 0.5-4.5 Al₂O₃ >10-23MgO >2.7-7 CaO 0.5-10 SrO >15-22 BaO 0.5-7 with MgO + CaO + SrO +BaO >20-35 SnO₂ 0-2 ZrO₂ 0-2 TiO₂ 0-2 CeO₂ 0-1.5 ZnO 0-1 Na₂O 0-2 K₂O0-2 with Na₂O + K₂O 0-3.


2. An aluminoborosilicate glass as claimed in claim 1, wherein saidglass has the following composition (in % by weight, based on oxide):SiO₂ 50-64 B₂O₃ 0.5-4.5 Al₂O₃ >10-22  MgO >2.7-7   CaO 0.5-10 SrO >15-22  BaO 0.5-7   with MgO + Cao + SrO + BaO >20-32  SnO₂   0-1.5ZrO₂   0-1.5 TiO₂   0-1.5 CeO₂ 0-1 ZnO 0-1 Na₂O 0-2 K₂O 0-2 with Na₂O +K₂O  0-3.


3. The aluminoborosilicate glass which contains alkaline earth metals asclaimed in claim 1, containing at least 2.5% by weight of BaO.
 4. Analuminoborosilicate glass as claimed in claim 1, wherein said glasscontains >26% by weight of MgO+CaO+SrO+BaO.
 5. An aluminoborosilicateglass as claimed in claim 1, wherein said glass contains at least 0.1%by weight of ZrO₂.
 6. An aluminoborosilicate glass as claimed in claim1, wherein said glass contains at least 0.1% by weight of SnO₂.
 7. Analuminoborosilicate glass as claimed in claim 1, wherein said glasscontains at least 0.1% by weight of CeO₂.
 8. An aluminoborosilicateglass as claimed in claim 1, wherein said glass contains at least 0.1%by weight of TiO₂.
 9. An aluminoborosilicate glass as claimed in claim1, wherein said glass additionally contains: As₂O₃ 0-1.5 Sb₂O₃ O-1.5 Cl⁻0-1.5 F⁻ 0-1.5 SO₄ ²⁻ 0-1.5 with As₂O₃ + Sb₂O₃ + Cl⁻+ F⁻+ SO₄ ²⁻ ≦1.5.


10. An aluminoborosilicate glass as claimed in claim 1, wherein saidglass is free of alkali metal oxides, apart from residues of refiningagent.
 11. An aluminoborosilicate glass as claimed in claim 1, whereinsaid glass has a coefficient of thermal expansion α_(20,300) of between4.5*10⁻⁶/K and 6.0*10⁻⁶/K and a transformation temperature T_(g)of >660° C.
 12. In a thin film photovoltaic device comprising aphotovoltaic thin film and substrate glass, the improvement wherein saidsubstrate glass comprises an aluminoborosilicate glass as claimed inclaim
 1. 13. In a lamp comprising a bulb glass containing a lightingelement, the improvement wherein said bulb glass comprises analkali-free aluminoborosilicate glass as claimed in claim 1 having acoefficient of thermal expansion α_(20/300) of between 4.5*10⁻⁶/K and5.0*10⁻⁶/K and a transformation temperature T_(g) of >715° C.
 14. Analuminoborosilicate glass as claimed in claim 2, wherein said glasscontains at least 2.5% by weight of BaO.
 15. An aluminoborosilicateglass as claimed in claim 14, wherein said glass contains >26% by weightof MgO+CaO+SrO+BaO.
 16. An aluminoborosilicate glass as claimed in claim15, wherein said glass contains at least 0.1% by weight of ZrO₂.
 17. Analuminoborosilicate glass as claimed in claim 16, wherein said glasscontains at least 0.1% by weight of SnO₂.
 18. An aluminoborosilicateglass as claimed in claim 17, wherein said glass contains at least 0.1%by weight of CeO₂.
 19. An aluminoborosilicate glass as claimed in claim18, wherein said glass is free of alkali metal oxides, apart fromresidues of refining agent.
 20. An aluminoborosilicate glass as claimedin claim 19, wherein said glass has a coefficient of thermal expansionα_(20/300) of between 4.5*10⁻⁶/K and 6.0*10⁻⁶/K and a transformationtemperature T_(g) of >660° C.
 21. An aluminoborosilicate glass asclaimed in claim 1, wherein said glass contains 0.5-9% by weight CaO.22. An aluminoborosilicate glass as claimed in claim 1, wherein saidglass contains at most <5% by weight BaO.
 23. An aluminoborosilicateglass as claimed in claim 1, wherein said glass contains <80% by weightof SiO₂+Al₂O₃+Na₂O+MgO.
 24. An aluminoborosilicate glass as claimed inclaim 1, wherein said glass contains at most <5% by weight ZnO.
 25. Analuminoborosilicate glass as claimed in claim 1, wherein said glasscontains at most 2.5-5% by weight BaO.